The invention relates to apparatus for mixing sand with one or more binder materials to form molding sand which can be used in casting machines for the making of cores or as a material for filling portions of molding boxes.
It is known to mix sand with one or more liquid and one or more solid binder materials so as to impart to the resulting molding sand a desirable consistency which is required in a casting or molding machine. The binder material can include one or more clayey substances, water, and/or flowable solid materials such as powdered wood, pulverulent iron oxide and/or others. As a rule, metered quantities of sand and of the constituents of binder material are introduced into a mixing chamber and the contents of the mixing chamber are thereupon mixed in order to convert them into a batch of molding sand which is ready for use in a casting machine. As a rule, the means for mixing includes a tool which is rotatable in the mixing chamber within a housing for metered quantities of sand and binder material, and the tool converts the contents of the chamber into a more or less homogeneous mixture constituting a batch of molding sand.
Heretofore known apparatus for producing molding sand, especially for the making of cores, are known as charge mixers, vibratory mixers or continuous (through flow) mixers.
A conventional charge mixer which is designed to make successive batches of molding sand is provided with a round housing wherein the height of the cylindrical chamber in the housing equals or exceeds the inner diameter of the housing. The bottom portion of the chamber contains a slowly rotating propeller which mixes the contents, primarily sand which is admitted in metered quantities. As a rule, the metering means comprises a scale or another suitable weighing device which determines the amount of sand to be admitted into the chamber prior to start of a batch- or charge-forming operation. Alternatively, sand can be admitted first into a vessel which is thereupon evacuated by dumping its contents into the housing of the charge-forming apparatus. A drawback of such apparatus is that the output is low and that they occupy a substantial amount of space, especially for the device or devices which are used for metering of sand and binder material. The metering device or devices are normally disposed at a level above the housing so that the overall height of such apparatus is often excessive. This holds especially true if, as is customary, the apparatus is disposed directly above a casting machine in which freshly formed batches of molding sand are put to use.
The mode of operation of the just described apparatus is such that a metered quantity of sand is admitted into the housing in a first step, and metered quantities of binder material are admitted thereafter on top of the metered quantity of sand. Such mode of operation cannot ensure a thorough intermixing of sand with all binder materials and, in addition, it takes a long time to convert the layers of superimposed constituents into a substantially homogeneous mixture which is ready to be admitted into a casting machine.
An additional drawback of such apparatus is that they cannot be readily cleaned upon the making of a batch. The housing is provided with a lateral door by way of which the batch of freshly formed molding sand is evacuated. It is unavoidable that a certain amount of molding sand will remain at the bottom of the housing as well as that particles of molding sand will adhere to the internal surfaces of the housing. The remnants of molding sand harden in the housing within a relatively short interval of time, normally within approximately 30 minutes. This interferes with the action of the mixing tool and reduces the effective volume of the housing. Moreover, remnants of molding sand in the housing can distort the composition of the next batch.
A conventional apparatus which employs a vibratory housing resembles the just discussed charge forming apparatus except that the mixing tool is replaced with a device which vibrates the entire housing. The metering devices are located at a level above the vibrating housing. The dimensions of such apparatus are rather small in order to ensure that oscillation of the housing will result in the making of a homogeneous batch of molding sand. Accordingly, the output of such apparatus is relatively low. In addition, large amounts of energy are required to agitate the entire housing. The quality of molding sand which is obtained in such apparatus is particularly unsatisfactory when the molding sand is to contain powdery binder material of low specific weight.
Continuous mixing apparatus employ a rotary feed screw which conveys sand and binder material through a housing. A drawback of such apparatus is that the groove or grooves of the feed screw are continuously filled with sand and binder material so that the feed screw necessitates frequent cleaning in order to prevent setting of molding sand in its groove or grooves. The cleaning operation must take place prior to admission of a fresh batch of sand and binder material in order to prevent changes in the composition of the freshly formed mixture. It is necessary to remove the feed screw from the housing at least once a week and to introduce it into a furnace in order to burn away hardened remnants of molding sand. The wear upon the feed screw and upon other parts of such apparatus is very pronounced, and the initial and maintenance costs are high.
German Auslegeschrift No. 1,284,946 of Hagele et al. discloses a mixer wherein a rotary vessel receives a main mixing unit and an auxiliary mixing unit. The latter is mounted on a support which serves to lift it out of the vessel or to lower it back into the interior of the vessel. The auxiliary mixing unit is used for preliminary intermixing of the contents of the vessel and is thereupon lifted out of the vessel prior to start of final mixing by the main mixing unit.
German Auslegeschrift No. 1,482,475 of Ronceray discloses a friction type mixer with a fixed vessel wherein mixing or grinding rollers are mounted on a vertically movable support for a stripping device in such a way that the rolls are lifted out of the vessel when the stripping device descends into the vessel and vice versa. The stripping device serves to promote evacuation of the mixture from the vessel by way of one or more bottom openings.
German Offenlegungsschrift No. 2,215,887 of Edwards discloses an apparatus wherein the mixing chamber receives a first charge of sand and binder material and a second charge of sand and a catalyst. The two charges are intermixed prior to admission by gravity feed into a core former or a molding box. The mixing chamber is cleaned upon completed evacuation of its contents